This proposal investigates the factors that contribute to sex-differences in molecular, cellular, and function properties in peripheral opioid receptor (POR) systems. We will focus on examining novel mechanisms by which sex-differences in peripherally applied opioid agonists are expressed in the context of inflammatory muscle pain conditions. The central hypotheses of this project are that sex-differences in POR mechanisms are expressed at multiple levels in primary afferent signaling process and injury or inflammation differentially impacts POR signaling between the sexes. Studies will determine whether there are sex-differences in (1) expression levels of the three subtypes of ORs, <, 4, and : ORs;(2) sub-cellular localizations of the three OR subtypes;and (3) the expression of major downstream targets, G-protein coupled and ATP dependent inward rectifying potassium channels (GIRK and KATP), under normal and inflammatory conditions. Each of these studies will be accompanied by behavioral tests to assess functional relevance of molecular and cellular changes. These studies bear high clinical significance since the pain and management involving masticatory muscles and TMJ, such as in TMJMD, are sexually dimorphic, and since there is increasing clinical as well as pre-clinical evidence that indicate PORs as potential therapeutic targets for treating various types of chronic pain conditions. Understanding mechanic bases for sex-differences in POR function will help develop sex-specific management strategies for persistent types of orofacial muscle pain. PUBLIC HEALTH RELEVANCE This project examines novel mechaisms that may underlie sexual dimorphism in peripheral opioid receptor (POR) effects in the context of inflammatory msucle pain conditions. PORs, namely, mu, delta and kappa ORs, are being increasingly recognized as important therapeutic targets that mediate anti-nociception and/or anti-hyperlagesia in inflammatory pain conditions without producing centrally-mediated side effects. Many types of chronic pain conditions such as temporomandibular joint muscle disorders (TMJMD) exhibit sexually dimorphic pain and analgesic responses. Therefore, outcomes of this project can offer important new insights for the development of mechanism-based sex-specific treatment alternatives that can be directed at the peripheral opioid receptor system to ameliorate persistent orofacial muscle pain.